1. Field of the Invention
The present invention relates to IC-based switching control circuits and switching power supply apparatuses including an IC-based switching control circuit.
2. Description of the Related Art
In switching power supply apparatuses, a switching control IC includes circuits for realizing various functions, such as an output control operation, an activation time control operation, an over-current protection operation, an over-voltage protection operation, a standby control operation, and a power factor improvement operation. With an increase in the number of these functions, higher capabilities are provided by the switching control IC.
To set the functions of a switching control IC to support the operational specifications of an application, a plurality of terminals are required for an interface with an external circuit, for each of the functions. Hence, an increase in the number of provided functions naturally results in an increase in the number of terminals. An increase in the number of terminals leads to an increase in the size of the package of the switching control IC and an increase in the unit cost of the IC.
If the number of terminals of a switching control IC is limited, the provided functions are limited. Hence, it becomes necessary to prepare various types of ICs corresponding to the specific functions and selectively use these types of ICs in accordance with the specifications or purposes. In this case, the number of the types of ICs is increased, resulting in increased complexity not only in the manufacturing process of ICs but also in the management of the ICs. This leads to an increase in the unit cost of the ICs.
In general, when more functions are required, the size of an IC is inevitably increased. However, in recent years, an increased number of functions have been required even in a small switching control circuit. At present, to lower the unit cost of semiconductor devices such as ICs, it is effective to produce a large number of few types of devices. Further, from the viewpoint of an IC package, the unit cost of an IC can be lowered when an IC is configured to have a small number of terminals and a small size.
Japanese Unexamined Patent Application Publication No. 2007-73954 discloses a switching control IC with an aim of reducing the number of terminals.
FIG. 1 is a circuit diagram of a switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2007-73954. Referring to FIG. 1, a switching power supply apparatus 101 includes a transformer 105 having a primary winding 127 and a secondary winding 129, a rectifying/smoothing circuit formed of a diode 117 and a capacitor 119, a feedback circuit formed of a Zener diode 121, a photocoupler 113, and a resistor 123, and an integrated circuit 103.
The integrated circuit 103 is connected to the primary winding 127. The integrated circuit 103 is a switching regulator IC including an internal switch coupled between a drain (D) terminal and a source (S) terminal of the integrated circuit 103.
During operation, a switch within the integrated circuit 103 controls transmission of energy from an input 107 to an output 109 via a transformer 105. A feedback signal is input to the integrated circuit 103 from the feedback circuit.
A multifunctional capacitor 111 is connected to a bypass terminal of the integrated circuit 103. The multifunctional capacitor 111 is used to provide a power supply coupling function of the integrated circuit 103 during a normal operation. An internal circuit within the integrated circuit 103, which receives a power or a bias current from the multifunctional capacitor 111, makes the circuit operate during a normal operation through control of the output 109.
During the initialization period of the integrated circuit 103, the multifunctional capacitor 111 is used to select the parameter/mode of the integrated circuit 103. During this initialization period, the parameter/mode of the integrated circuit is selected.
One of the functions of a switching power supply apparatus is a soft start function. This is a function of gradually increasing the on-period of an output pulse for driving an FET in accordance with a voltage of a soft start control terminal (SS terminal) at the time of activating the power supply. The speed with which a voltage of the SS terminal changes is set by the capacitance of a capacitor externally connected to the SS terminal. In other words, the speed with which the voltage of the capacitor voltage at the time of activation increases determines the speed with which the on-period is increased, whereby a soft start period is determined. Hence, when the capacitance is large, the voltage increases at a low speed and the soft start period becomes long (the on-period is increased at a slow pace), and when the capacitance is small, the voltage increases at a high speed and the soft start period becomes short (the on-period is increased at a fast pace).
However, such a soft start control terminal (SS terminal) controls only the period of a soft start by controlling the speed with which the voltage of the soft start terminal is increased, and cannot determine the maximum value of the on-pulse period. Hence, a terminal different from the SS terminal is required to control the maximum on-pulse period, thereby limiting power. As a result, the number of terminals of the IC is increased, which prevents a reduction in the size of the IC and increases the unit cost of the IC. In other words, there is a problem in that the size of the package of a switching control IC is increased and the unit cost of the IC is increased.